Method of fabricating FinFETs

ABSTRACT

The disclosure relates to a method of fabricating a semiconductor device including forming a patterned hardmask layer over a substrate comprising a major surface. The method further includes forming a plurality of first trenches and a plurality of second trenches performed at an electrostatic chuck (ESC) temperature between about 90° C. to 120° C. in the substrate. The plurality of first trenches have a first width and extend downward from the substrate major surface to a first height, and the plurality of second trenches have a second width less than first width and extend downward from the substrate major surface to a second height greater than the first height.

PRIORITY CLAIM

The present application is a divisional of U.S. application Ser. No. 13/407,507, filed Feb. 28, 2012, which is incorporated by reference herein in its entirety.

TECHNICAL FIELD

The disclosure relates to integrated circuit fabrication, and more particularly to a method of fabricating fin field effect transistors (FinFETs).

BACKGROUND

As the semiconductor industry has progressed into nanometer technology process nodes in pursuit of higher device density, higher performance, and lower costs, challenges from both fabrication and design issues have resulted in the development of three-dimensional designs, such as a fin field effect transistor (FinFET). A typical FinFET is fabricated with a thin vertical “fin” (or fin structure) extending from a substrate formed by, for example, etching away a portion of a silicon layer of the substrate. The channel of the FinFET is formed in this vertical fin. A gate is provided over (e.g., wrapping) the fin. Having a gate on both sides of the channel allows gate control of the channel from both sides. In addition, strained materials in source/drain (S/D) portions of the FinFET utilizing selectively grown silicon germanium (SiGe) may be used to enhance carrier mobility.

However, there are challenges to implementation of such features and processes in complementary metal-oxide-semiconductor (CMOS) fabrication. For example, high stress of shallow trench isolation (STI) oxide causes fin deformation of the FinFET, thereby degrading the device performance.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale and are used for illustration purposes only. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIG. 1 is a flowchart of a method of fabricating a FinFET according to various aspects of the present disclosure;

FIGS. 2A-2G are cross-sectional views of a FinFET at various stages of fabrication according to various embodiment of the present disclosure; and

FIGS. 3A-3B are cross-sectional views of two example conventional FinFETs at stages of fabrication comparable to that of FIG. 2C and FIG. 2D, respectively.

DESCRIPTION

It is understood that the following disclosure provides many different embodiments, or examples, for implementing different features of the invention. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

Referring to FIG. 1, illustrated is a flowchart of a method 100 of fabricating a fin field effect transistor (FinFET) according to various aspects of the present disclosure. The method 100 begins with step 102 in which a patterned hardmask layer is formed over a substrate comprising a major surface. The method 100 continues with step 104 in which a plurality of first trenches and a plurality of second trenches are formed. The formation of the plurality of first trenches and the plurality of second trenches is performed at an electrostatic chuck (ESC) temperature (i.e., the ESC is set to have a temperature) between about 90° C. to 120° C., the plurality of first trenches having a first width and extending downward from the substrate major surface to a first height, the plurality of second trenches having a second width less than first width and extending downward from the substrate major surface to a second height greater than the first height. The discussion that follows illustrates embodiments of FinFETs that can be fabricated according to the method 100 of FIG. 1.

FIGS. 2A-2G are cross-sectional views of a fin field effect transistor (FinFET) 200 at various stages of fabrication according to various embodiment of the present disclosure. As employed in the present disclosure, the FinFET 200 refers to any fin-based, multi-gate transistor. The FinFET 200 may be included in a microprocessor, memory cell, and/or other integrated circuit (IC). It is noted that the method of FIG. 1 does not produce a completed FinFET 200. A completed FinFET 200 may be fabricated using complementary metal-oxide-semiconductor (CMOS) technology processing. Accordingly, it is understood that additional processes may be provided before, during, and after the method 100 of FIG. 1, and that some other processes may only be briefly described herein. Also, FIGS. 2A through 2G are simplified for a better understanding of the concepts of the present disclosure. For example, although only the FinFET 200 is depicted in FIGS. 2A-2G, it is understood the IC may comprise a number of other devices comprising resistors, capacitors, inductors, fuses, etc.

Referring to FIG. 2A, a substrate 202 comprising a major surface 202 s is provided, wherein the substrate 202 comprises an sparse region (also being referred to as an “iso region”) 202 a and a dense region 202 b. The dense region refers to a region where core devices would be formed, because the fin structures are densely formed. The sparse or iso region refers to a peripheral region where I/O devices would be formed, because the fin structures are not densely formed compared to the dense region.

In the depicted embodiment, the iso region 202 a may be configured for an input/output (I/O) device, while the dense region 202 b may be configured for a core device. In at least one embodiment, the substrate 202 comprises a crystalline silicon substrate (e.g., wafer). The substrate 202 may comprise various doped regions depending on design requirements (e.g., p-type substrate or n-type substrate). In some embodiments, the doped regions may be doped with p-type or n-type dopants. For example, the doped regions may be doped with p-type dopants, such as boron or BF₂; n-type dopants, such as phosphorus or arsenic; and/or combinations thereof. The doped regions may be usable for forming an n-type FinFET, or alternatively for forming a p-type FinFET.

In some alternative embodiments, the substrate 202 may be made of some other suitable elemental semiconductor, such as diamond or germanium; a suitable compound semiconductor, such as gallium arsenide, silicon carbide, indium arsenide, or indium phosphide; or a suitable alloy semiconductor, such as silicon germanium carbide, gallium arsenic phosphide, or gallium indium phosphide. Further, the substrate 202 may include an epitaxial layer (epi-layer), may be strained for performance enhancement, and/or may include a silicon-on-insulator (SOI) structure.

The fins are formed by etching into the substrate 202. In at least one embodiment, a pad layer 204 a and a mask layer 204 b are formed on the semiconductor substrate 202. In the depicted embodiment, the pad layer 204 a and mask layer 204 b are collectively referred as a hardmask layer 204. The pad layer 204 a may be a thin film comprising silicon oxide formed, for example, using a thermal oxidation process. The pad layer 204 a may act as an adhesion layer between the semiconductor substrate 202 and mask layer 204 b. The pad layer 204 a may also act as an etch stop layer for etching the mask layer 204 b. In some embodiments, the mask layer 204 b is formed of silicon nitride, for example, using low-pressure chemical vapor deposition (LPCVD) or plasma enhanced chemical vapor deposition (PECVD). The mask layer 204 b is used as a hardmask during subsequent photolithography processes. A photo-sensitive layer 206 is formed on the mask layer 204 b and is then patterned, forming a plurality of first openings 208 a and a plurality of second opening 208 b in the photo-sensitive layer 206. In the depicted embodiment, the plurality of the first openings 208 a has a first width W₁, while the plurality of the second opening 208 b has a second width W₂ less than the first width W₁. In other words, the plurality of the first openings 208 a is over the iso region 200 a, while the plurality of second opening 208 b is over the dense region 200 b.

Referring to FIG. 2B, subsequent to the formation of the plurality of the first openings 208 a and the plurality of the second opening 208 b in the photo-sensitive layer 206, the mask layer 204 b and pad layer 204 a are etched through the plurality of the first openings 208 a and the plurality of the second opening 208 b to forming a patterned hardmask layer 204, thereby exposing underlying semiconductor substrate 202. The patterned hardmask layer 204 comprises a plurality of third openings 210 a has the first width W₁ and a plurality of fourth opening 210 b has a second width W₂ less than the first width W₁.

In at least one embodiment, the etching step may be performed using a dry etching process, for example, the dry etching process may be performed at an electrostatic chuck (ESC) temperature between about 45° C. to 65° C. and under a pressure of about 2 mTorr to 20 mTorr and a source power of about 500 to 1000 W, using a chemical selected from NF₃, CF₄, and SF₆ as an etching gas. The photo-sensitive layer 206 is then removed.

The process steps up to this point have provided the substrate 202 having a patterned hardmask layer 204 over a substrate 202 comprising a major surface 202 s, thereby exposing underlying semiconductor substrate 202. The exposed semiconductor substrate 202 is then etched to form trenches extending downward from the substrate major surface 202 s. Portions of the semiconductor substrate 202 between trenches form semiconductor fins.

FIG. 3A is a cross-sectional view of a conventional FinFETs at a stage of fabrication comparable to that of FIG. 2C. Conventionally, the etching step may be performed using a dry etching process. Due to more polymer formation on surface of trenches through the plurality of the fourth openings 210 b during the dry etching process, a fourth height H₄ of a plurality of fourth trenches 312 b in the dense region 202 b formed through the plurality of the fourth openings 210 b is less than a third height H₃ of a plurality of third trenches 312 a in the iso region 202 a formed through the plurality of the third opening 210 a. In at least one configuration, portions of the semiconductor substrate 202 between the plurality of third trenches 312 a and the plurality of fourth trenches 312 b form a plurality of semiconductor fins, such as outer fins 314 a and 314 e. In another configuration, portions of the semiconductor substrate 202 between the plurality of fourth trenches 312 b form a plurality of semiconductor fins, such as inner fins 314 b, 314 c, and 314 d. In the configuration depicted in FIG. 3A, a subset of the semiconductor fins 314 of one FinFET may comprise the outer fins 314 a and 314 e and inner fins 314 b, 314 c, and 314 d.

FIG. 3B is a cross-sectional view of a conventional FinFETs at a stage of fabrication comparable to that of FIG. 2D. However, after filling the plurality of third trenches 312 a and the plurality of fourth trenches 312 b with a shallow trench isolation (STI) oxide 316, stress on inner fins 314 b, 314 c, and 314 d adjacent to similar trenches 312 b with same height H₃ is low, while stress on outer fins 314 a and 314 e adjacent to different trenches 312 a and 312 b with different heights H₃ and H₄ is high. The high stress of the STI oxide 316 may deform the outer fins 314 a and 314 e (shown in FIG. 3B), thereby degrading the device performance.

Accordingly, the processing discussed below with reference to FIGS. 2C and 2D may etch the semiconductor substrate 202 with less polymer formation on surface of trenches through the plurality of the fourth openings 210 b to increase trench height in the dense region 202 b. This can help reduce stress on outer fins, thereby fabricating uniform fins of the FinFET 200 and thus improving device performance.

In the depicted embodiment, after forming the patterned hardmask layer 204 to expose underlying semiconductor substrate 202, the exposed semiconductor substrate 202 is then etched to form trenches extending downward from the substrate major surface 202 s. FIG. 2C shows the FinFET 200 of FIG. 2B after forming a plurality of first trenches 212 a and a plurality of second trenches 212 b. The plurality of first trenches 212 a has the first width W₁ and extends downward from the substrate major surface 202 s to a first height H₁. The plurality of second trenches 212 b has a second width W₂ less than first width W₁ and extends downward from the substrate major surface 202 s to a second height H₂ greater than the first height H₁. Because the second height H₂ is greater than the first height H₁, this can help reduce stress on outer fins after STI oxide formation, thereby fabricating uniform fins of the FinFET 200 and thus improving device performance.

In at least one embodiment, the second width W₂ ranges from about 300 Å to about 1500 Å. In some embodiments, a ratio of the second width W₂ to the first width W₁ is from about 0.1 to about 0.5. In another embodiment, the second height H₂ may range from about 1200 Å to about 2500 Å. In another embodiment, a ratio of the second height H₂ to the first height H₁ is from about 1.05 to about 1.25. One skilled in the art will realize, however, that the dimensions and values recited throughout the descriptions are merely examples, and may be changed to suit different scales of integrated circuits.

In at least one embodiment, the plurality of first trenches 212 a may be strips (viewed from in the top of the FinFET 200) parallel to each other and spaced with respect to each other. A first space S₁ between adjacent first trenches 212 a defines a first fin 214 f. In the depicted embodiment, the first fin 214 f comprises substantially vertical sidewalls. In some alternative embodiment, the first fin 214 f comprises tapered sidewalls (not shown).

In another embodiment, the plurality of second trenches 212 b may be strips (viewed from in the top of the FinFET 200) parallel to each other and spaced with respect to each other. A second space S₂ between adjacent second trenches 212 b defines a subset of second fins, such as inner fins 214 b, 214 c and 214 d. In the depicted embodiment, each of the inner fins 214 b, 214 c and 214 d comprises substantially vertical sidewalls. In some alternative embodiment, each of the inner fins 214 b, 214 c and 214 d, comprises tapered sidewalls (not shown). In some embodiments, the second space S₂ is equal to or less than the first space S₁. In some embodiments, a ratio of the second space S₂ to the first space S₁ is from about 0.5 to about 1.

Further, a third space S₃ between adjacent first trench 212 a and second trench 212 b defines another subset of second fins, such as outer fins 214 a and 214 e. In the depicted embodiment, each of the outer fins 214 a and 214 e comprises substantially vertical sidewalls. In some alternative embodiment, each of the outer fins 214 a and 214 e comprises tapered sidewalls (not shown). In some embodiments, the third space S₃ is substantially equal to the second space S₂. In some embodiments, a ratio of the third space S₃ to the second space S₂ is from about 0.9 to about 1.1. In the depicted embodiment, a subset of the second fins 214 of the FinFET 200 may comprise the outer fins 214 a and 214 e and inner fins 214 b, 214 c, and 214 d.

In at least one embodiment, the etching step may be performed using a dry etching process, for example, the dry etching process may be performed at an electrostatic chuck (ESC) temperature between about 90° C. to 120° C. and under a pressure of about 2 mTorr to 20 mTorr, and under a source power of about 500 to 1000 W and a pulse bias voltage, using a chemical selected from NF₃, CF₄, and SF₆ as an etching gas. Next, a cleaning may be performed to remove a native oxide of the semiconductor substrate 202. The cleaning may be performed using diluted hydrofluoric (DHF) acid.

Liner oxide (not shown) is then optionally formed in the plurality of first trenches 212 a and the plurality of first trenches 212 b. In an embodiment, liner oxide may be a thermal oxide having a thickness ranging from about 20 Å to about 500 Å. In some embodiments, liner oxide may be formed using in-situ steam generation (ISSG) and the like. The formation of liner oxide rounds corners of the trenches 212 a and 212 b, which reduces the electrical fields, and hence improves the performance of the resulting integrated circuit.

FIG. 2D depicts the resulting structure after filling the trenches 212 a and 212 b with a dielectric material 216. In some embodiments, the dielectric material 216 may include silicon oxide, and hence is also referred to as oxide 216 in the present disclosure. In some embodiments, other dielectric materials, such as silicon nitride, silicon oxynitride, fluoride-doped silicate glass (FSG), or a low-K dielectric material, may also be used. In an embodiment, the oxide 216 may be formed using a high-density-plasma (HDP) CVD process, using silane (SiH₄) and oxygen (O₂) as reacting precursors. In other embodiment, the oxide 216 may be formed using a sub-atmospheric CVD (SACVD) process or high aspect-ratio process (HARP), wherein process gases may comprise tetraethylorthosilicate (TEOS) and ozone (O₃). In yet some other embodiments, the oxide 216 may be formed using a spin-on-dielectric (SOD) process, such as hydrogen silsesquioxane (HSQ) or methyl silsesquioxane (MSQ). Because the second height H₂ is greater than the first height H₁, this can help reduce stress on outer fins after STI oxide 216 formation, thereby fabricating uniform fins of the FinFET 200 and thus improving device performance.

Referring to FIG. 2E, after filling the trenches 212 a and 212 b with a dielectric material 216, a chemical mechanical polish is then performed, followed by the removal of the mask layer 204 b and pad layer 204 a. The CMP process and the removal of the mask layer 204 b and pad layer 204 a produce the structure shown in FIG. 2E. In at least one embodiment, the mask layer 204 b is formed of silicon nitride, the mask layer 204 b may be removed using a wet process using hot H₃PO₄, while the pad layer 204 a may be removed using diluted HF acid, if formed of silicon oxide. In some alternative embodiments, the removal of the mask layer 204 b and pad layer 204 a may be performed after the recessing of the insulation layers 216, which recessing step is shown in FIG. 2F.

As shown in FIG. 2F, the insulation layers 216 are recessed by an etching step, resulting in recesses 218. In at least one embodiment, the etching step may be performed using a wet etching process, for example, by dipping the substrate 202 in hydrofluoric acid (HF). In another embodiment, the etching step may be performed using a dry etching process, for example, the dry etching process may be performed using CHF₃ or BF₃ as etching gases.

The remaining insulation layer 216 comprises a top surface 216 t extending downward from the substrate major surface 202 s to a fifth height H₅ less than the first height H₁ and second height H₂, whereby upper portion 224 f of the first fin 214 f and upper portion 224 b of the second fin 214 b extent beyond the top surface 216 t of the insulation layer 216. In at least one embodiment, the fifth height H₅ may be between about 300 Å to about 1000 Å, although it may also be greater or smaller. In the depicted embodiment, the upper portion 224 f of the first fin 214 f may comprise a top surface 224 f_1 and sidewalls 224 f_2 and 224 f_3, while the upper portion 224 b of the second fin 214 b comprise a top surface 224 b_1 and sidewalls 224 b_2 and 224 b_3.

Referring to FIG. 2G, as well as FIGS. 2C and 2F, following formation the recesses 218, a first gate stack 220 a is formed over the upper portion 224 f of the first fin 214 f, while a second gate stack 220 b is formed over the upper portion 224 b of the second fin 214 b. The first gate stack 220 a and second gate stack 220 b are spaced from each other by a dielectric 230. In some embodiments, each of the first gate stack 220 a and second gate stack 220 b comprises a gate dielectric layer 226 and a gate electrode layer 228 over the gate dielectric layer 226. Thus, the FinFET 200 comprises a first FinFET 200 a and a second FinFET 200 b. In at least one embodiment, the first FinFET 200 a comprises a plurality of first trenches 212 a having a first width W₁ and extending downward from the substrate major surface 202 s to a first height H₁, wherein a first space S₁ between adjacent first trenches 212 a defines a first fin 214 f; a first gate dielectric 226 a on a top surface 224 f_1 and sidewalls 224 f_2 and 224 f_3 of the first fin 214 f; and a first gate electrode 228 a on the first gate dielectric 226 a. In another embodiment, the second FinFET 200 b comprises a plurality of second trenches 212 b having a second width W₂ less than first width W₁ and extending downward from the substrate major surface 202 s to a second height H₂ greater than the first height H₁, wherein a second space S₂ between adjacent second trenches 212 b defines a second fin 214 b; a second gate dielectric 226 b on a top surface 224 b_1 and sidewalls 224 b_2 and 224 b_3 of the second fin 214 b; and a second gate electrode 228 b on the second gate dielectric 226 b.

In the depicted embodiment, a gate dielectric 226 is formed to cover the upper portion 224 f of the first fin 214 f and the upper portion 224 b of the second fin 214 b. In some embodiments, the gate dielectric 226 may include silicon oxide, silicon nitride, silicon oxynitride, or high-k dielectrics. High-k dielectrics comprise metal oxides. Examples of metal oxides used for high-k dielectrics include oxides of Li, Be, Mg, Ca, Sr, Sc, Y, Zr, Hf, Al, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and mixtures thereof. In the present embodiment, the gate dielectric 226 is a high-k dielectric layer with a thickness in the range of about 10 to 30 angstroms. The gate dielectric 226 may be formed using a suitable process such as atomic layer deposition (ALD), chemical vapor deposition (CVD), physical vapor deposition (PVD), thermal oxidation, UV-ozone oxidation, or combinations thereof. The gate dielectric 226 may further comprise an interfacial layer (not shown) to reduce damage between the gate dielectric 226 and the upper portion 224 f of the first fin 214 f (and the upper portion 224 b of the second fin 214 b). The interfacial layer may comprise silicon oxide.

The gate electrode 228 is then formed on the gate dielectric 226. In at least one embodiment, the upper portion 224 f of the first fin 214 f may be used to form a separate FinFET, such as the first FinFET 200 a, which may be a portion of an I/O device. In some alternative embodiments, the gate electrode 228 may cover the upper portion 224 b of more than one semiconductor fin 214 b, so that the resulting FinFET comprises more than one fin, such as the second FinFET 200 b, which may be a portion of a core device.

In some embodiments, the gate electrode 228 may comprise a single layer or multilayer structure. In the present embodiment, the gate electrode 228 may comprise poly-silicon. Further, the gate electrode 228 may be doped poly-silicon with the uniform or non-uniform doping. In some alternative embodiments, the gate electrode layer 228 may include a metal such as Al, Cu, W, Ti, Ta, TiN, TiAl, TiAlN, TaN, NiSi, CoSi, other conductive materials with a work function compatible with the substrate material, or combinations thereof. In the present embodiment, the gate electrode 228 comprises a thickness in the range of about 30 nm to about 60 nm. The gate electrode 228 may be formed using a suitable process such as ALD, CVD, PVD, plating, or combinations thereof.

It is understood that the FinFET 200 may undergo further CMOS processes to form various features such as source/drain regions, contacts/vias, interconnect metal layers, dielectric layers, passivation layers, etc. Thus, Applicant's method can help reduce stress on outer fins, thereby fabricating uniform fins of the FinFET 200 and thus improving device performance.

In accordance with an embodiment, a method of fabricating a semiconductor device includes forming a patterned hardmask layer over a substrate comprising a major surface. The method further includes forming a plurality of first trenches and a plurality of second trenches performed at an electrostatic chuck (ESC) temperature between about 90° C. to 120° C. in the substrate. The plurality of first trenches have a first width and extend downward from the substrate major surface to a first height, and the plurality of second trenches have a second width less than first width and extend downward from the substrate major surface to a second height greater than the first height.

In accordance with another embodiment, a method of making a fin field effect transistor (FinFET) includes forming a plurality of first trenches having a first width and a first depth in a substrate, where each first trench of the plurality of first trenches extends downward from a surface of the substrate to a first height. A first space between adjacent first trenches defines a first fin. The method further includes forming a plurality of second trenches having a second width and a second depth in the substrate, where the second width is less than the first width. Further, each second trench of the plurality of second trenches extends downward from the surface of the substrate to a second height greater than the first height. A second space between adjacent second trenches defines a second fin. Additionally, the method includes forming a first isolation structure partially filling each first trench of the plurality of first trenches. The method further includes forming a second isolation structure partially filling each second trench of the plurality of second trenches, where a difference between a height of the first isolation structure and the first depth is substantially equal to a difference between a height of the second isolation structure and the second depth.

In accordance with another embodiment, a method of making a fin field effect transistor (FinFET) includes forming a plurality of first trenches having a first width and a first depth in a substrate, where each first trench of the plurality of first trenches extends downward from a surface of the substrate to a first height. A first space between adjacent first trenches defines a first fin. The method further includes forming a plurality of second trenches having a second width and a second depth in the substrate, where the second width is less than the first width. Further, each second trench of the plurality of second trenches extends downward from the surface of the substrate to a second height greater than the first height. A second space between adjacent second trenches defines a second fin. Additionally, the method includes forming a first isolation structure partially filling each first trench of the plurality of first trenches. The method further includes forming a second isolation structure partially filling each second trench of the plurality of second trenches, where a difference between a height of the first isolation structure and the first depth is substantially equal to a difference between a height of the second isolation structure and the second depth. Moreover, the method includes forming a first gate electrode over a top surface and sidewalls of the first fin. The method further includes forming a second gate electrode over a top surface and sidewalls of the second fin.

While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements. 

What is claimed is:
 1. A method of making a fin field effect transistor (FinFET) comprising: forming a plurality of first trenches having a first width and a first depth in a substrate, each first trench of the plurality of first trenches extending downward from a surface of the substrate to a first height, wherein a first space between adjacent first trenches defines a first fin; forming a plurality of second trenches having a second width and a second depth in the substrate, the second width less than the first width, each second trench of the plurality of second trenches extending downward from the surface of the substrate to a second height greater than the first height, wherein a second space between adjacent second trenches defines a second fin; forming a first isolation structure partially filling each first trench of the plurality of first trenches; and forming a second isolation structure partially filling each second trench of the plurality of second trenches, wherein a difference between a height of the first isolation structure and the first depth is substantially equal to a difference between a height of the second isolation structure and the second depth.
 2. The method of claim 1, wherein forming each of the plurality of first and second trenches comprises using a chemical selected from NF₃, CF₄, and SF₆ as an etching gas.
 3. The method of claim 1, wherein each of the plurality of first and second trenches are formed under a pressure of about 2 mTorr to 20 mTorr.
 4. The method of claim 1, wherein each of the plurality of first and second trenches are formed under a source power of about 500 W to 1000 W.
 5. The method of claim 1, wherein each of the plurality of first and second trenches are formed under a pulse bias voltage.
 6. The method of claim 1, wherein forming the first and second isolation structure further comprises forming a liner oxide in each of the plurality of first and second trenches.
 7. The method of claim 1, further comprising forming a first gate stack over the first fin, and forming a second gate stack over the second fin.
 8. The method of claim 7, further comprising forming a dielectric layer between the first gate stack and the second gate stack.
 9. A method of making a fin field effect transistor (FinFET) comprising: forming a plurality of first trenches having a first width and a first depth in a substrate, each first trench of the plurality of first trenches extending downward from a surface of the substrate to a first height, wherein a first space between adjacent first trenches defines a first fin; forming a plurality of second trenches having a second width and a second depth in the substrate, the second width less than the first width, each second trench of the plurality of second trenches extending downward from the surface of the substrate to a second height greater than the first height, wherein a second space between adjacent second trenches defines a second fin; forming a first isolation structure partially filling each first trench of the plurality of first trenches; forming a second isolation structure partially filling each second trench of the plurality of second trenches, wherein a difference between a height of the first isolation structure and the first depth is substantially equal to a different between a height of the second isolation structure and the second depth; forming a first gate electrode over a top surface and sidewalls of the first fin; and forming a second gate electrode over a top surface and sidewalls of the second fin.
 10. The method of claim 9, wherein forming each of the plurality of first and second trenches comprises using a chemical selected from NF₃, CF₄, and SF₆ as an etching gas.
 11. The method of claim 9, wherein each of the plurality of first and second trenches are formed under a pressure of about 2 mTorr to 20 mTorr.
 12. The method of claim 9, wherein each of the plurality of first and second trenches are formed under a source power of about 500 W to 1000 W.
 13. The method of claim 9, wherein each of the plurality of first and second trenches are formed under a pulse bias voltage.
 14. The method of claim 9, wherein forming the first and second isolation structure further comprises forming a liner oxide in each of the plurality of first and second trenches.
 15. The method of claim 9, further comprising forming a dielectric layer between the first gate electrode and the second gate electrode. 